1. Field of the Art:
The present invention relates to an electrophotographic device such as laser printer.
2. Background of the Prior Art:
A known electrophotographic device is shown in FIG. 2 and includes a photosensitive element 12 having a photosensitive film 14, such as amorphous selenium, placed over an electrically conductive member 16, such as aluminum. The photosensitive element 12 is charged by an electrifier 18 to an electrostatic potential Vo and then selectively exposed to laser beams 26.
Photosensitive film 14 is a dielectric in its unexposed state, but loses much of its dielectric qualities after exposure to laser beam 26, thus exhibiting properties of a conductor and the accumulated charge on the exposed areas flow to ground via conductive member 16. This results in exposed areas having a reduced potential Vr that is lower than the electrostatic potential Vo. The whole unexposed area is typically referred to as the electrostatic latent image.
The electrostatic latent image is then developed into a printed visible image using a developing machine, not shown in FIG. 2.
The values of Vo, Vr, and, Vo-Vr are extremely important in determining the quality of the print, and must be kept at constant values. However, these values have, in the past, been very dependent on the physical properties of photosensitive element 12. Also, temperature variations have caused deviations from constant values as shown in FIG. 3. The result of this is that lower quality printed images are obtained.
One known manner to keep these potentials constant measures the electrostatic potential Vo and uses this measured potential to vary the voltage applied by electrifier 18 shown in FIG. 2. This results in a constant electrostatic potential Vo, but the reduced potential Vr can still vary a great deal. For, example, as shown in FIG. 4, for varying temperatures, the electrostatic potential Vo is constant, but reduced potential Vr changes considerably.
Attempts have also been made to use materials that will cause these potential maintain a more constant value. However, the result of these attempts has been very expensive machinery that still does not adequately keep these potentials at a constant value.
Thus, an electrophotographic device that can maintain both a constant electrostatic potential Vo and a constant reduced potential Vr is needed.